The invention generally relates to planar elements such as doctor blades and top plates, for use in papermaking machines, and relates in particular to doctor blades and top plates that provide improved stiffness and performance.
Doctor blades and top plates for use in doctor blade holders, are generally employed for processing moving surfaces in manufacturing systems involving rolls, webs or sheets such as in papermaking systems.
The processing of the moving surfaces is generally done by a doctor blade that is held by a doctor blade holder, which may include a top plate. The doctor blade holder works with the doctor blade to apply the working edge of the doctor blade to the surface being processed. FIG. 1 for example, shows a prior art doctoring apparatus 10 that is adjacent to the surface of a roll 12. The roll rotates about an axis A1, and the doctoring apparatus includes a doctorback 14 that is rotatable about an axis A2, which is parallel to the axis A1. A doctor blade holder 16 is shown supported on a beam 18, which forms part of the doctorback. The doctor blade holder 16 includes has top plate 20 and a bottom plate 22 that are joined by a mounting and adjustment assembly. A doctor blade 24 is received within a lower jaw opening 26 on the underside of the top plate 20.
With further reference to FIG. 2, the mounting and adjustment assembly includes a plurality of top plate brackets 28 and a plurality of bottom plate brackets 30 that are mutually joined together by a pivot rod 32. The mounting and adjustment assembly also includes a loading tube 34 and an unloading tube 36 that may each be alternately increased or decreased in size by adjusting an amount of fluid within each tube to effect a limited rotation of the top plate with respect to the axis A3 that is the central axis of the pivot rod 32. This limited rotation permits the doctor blade 24 to engage the roll 12 to effect doctoring, or to disengage the roll 12. A piston/cylinder unit 38 acts via a crank arm 40 to rotate the doctorback 14 about axis A2 in order to provide gross positioning of the doctor blade 24 near and away from the roll surface.
The doctor blade may be formed of metal (for high stiffness and low thickness), plastic (for better conformance to the surface at a cost of being thicker), or a reinforced plastic. U.S. Pat. No. 4,549,933, for example discloses the use of synthetic material (e.g., fiber, some of which are oriented) in forming layers that are laminated together to form a doctor blade. U.S. Pat. No. 4,549,933 in fact, states that high stiffness and strength in the machine direction permits required blade loading with low deflection and without unacceptable increases in blade thickness (which translate into power losses), while lower stiffness in the cross machine direction allows the blade to confirm to roll surface irregularities without large changes in the required blade loading.
U.S. Patent Application Publication No. 2009/0208706 discloses a method of making a doctor blade from a thick needled felt consisting of polyamide and co-polyester fibres with a polyurethane surface coating. U.S. Pat. No. 6,977,029 discloses a doctor blade for a shoe press application made from a woven fabric base layer with plural fibrous batt layers needled punched into it. The structure is also made from polyamide, aromatic polyamide and polyester fibres.
Certain prior art doctor blades and top plates are made by applying several individual layers of 2-Dimensional reinforcement fabric together with a polymeric resin. The 2-D reinforcement fabrics used typically comprise glass fibres, carbon fibres or combinations of both. Glass fabrics impart the doctor blades with cleaning and abrasive properties, whilst carbon reinforcement is used to improve the wear life of the doctor blade and strength and stiffness of a top plate. Different combinations of both are used depending on the property and performance requirement of a particular doctor blade or top plate. For example, 8 layers of 200 gsm plain weave 2-D glass fabric may be bonded together to produce a doctor blade of thickness 1.5 mm, as shown for example in FIG. 3.
Fabric reinforcement containing aramid fibres may also be included to impart the doctor blades and top plates with increased impact resistance. Micron sized fillers are also commonly used to further reinforce the resin and give the blades additional property enhancements. For example silicon carbide is commonly used to give doctor blades additional abrasive properties.
Heat and pressure have been used to cure the polymeric resin and chemically bond the individual 2-D reinforcement layers together. Under the extreme conditions often experienced on a paper machine however, including, contact with abrasive and aggressive paper fillers, e.g. calcium carbonate and titanium dioxide, impact with stickies and exposure to petrochemical based roll cleaning solvents, the chemical bonds can fail causing the individual layers to open up and delaminate, ultimately resulting in failure, as paper stock fibres penetrate into the body of the blade or Top Plate as shown, for example in FIG. 4.
There remains a need therefore, for a planar element such as a doctor blade, for use in a papermaking machine, wherein the doctor blade provides improved performance and durability.